46.4.2.2 Receiver bit ordering
When S2[MSBF] is set, the receiver operates such that the first bit received after the start
bit is the MSB of the dataword. Similarly, the bit received immediately preceding the
parity bit, or the stop bit if parity is not enabled, is treated as the LSB for the dataword.
All necessary bit ordering is handled automatically by the module. Therefore, the format
of the data read from receive data buffer is completely independent of S2[MSBF].
46.4.2.3 Character reception
During UART reception, the receive shift register shifts a frame in from the
unsynchronized receiver input signal. After a complete frame shifts into the receive shift
register, the data portion of the frame transfers to the UART receive buffer. The receive
data buffer is accessible via the D and C3[T8] registers. S1[RDRF] is set if the number of
resulting datawords in the receive buffer is equal to or greater than the number indicated
by RWFIFO[RXWATER]. If the C2[RIE] is also set, RDRF generates an RDRF interrupt
request. Alternatively, by programming C5[RDMAS], a DMA request can be generated.
When C7816[ISO_7816E] is set/enabled and C7816[TTYPE] = 0, character reception
operates slightly differently. Upon receipt of the parity bit, the validity of the parity bit is
checked. If C7816[ANACK] is set and the parity check fails, or if INIT and the received
character is not a valid initial character, then a NACK is sent by the receiver. If the
number of consecutive receive errors exceeds the threshold set by
ET7816[RXTHRESHOLD], then IS7816[RXT] is set and an interrupt generated if
IE7816[RXTE] is set. If an error is detected due to parity or an invalid initial character,
the data is not transferred from the receive shift register to the receive buffer. Instead, the
data is overwritten by the next incoming data.
When the C7816[ISO_7816E] is set/enabled, C7816[ONACK] is set/enabled, and the
received character results in the receive buffer overflowing, a NACK is issued by the
receiver. Additionally, S1[OR] is set and an interrupt is issued if required, and the data in
the shift register is discarded.
46.4.2.4 Data sampling
The receiver samples the unsynchronized receiver input signal at the RT clock rate. The
RT clock is an internal signal with a frequency 16 times the baud rate. To adjust for baud
rate mismatch, the RT clock (see the following figure) is re-synchronized:
Chapter 46 Universal Asynchronous Receiver/Transmitter (UART)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
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